[Crawford]

I bet you there's some location in Rhein-Ruhr that has highest radius population in Europe.

But as others have mentioned, this is just waaay too broad geography to really ascertain regional population. In the Ruhr, you can be 5 km out of a major city like Essen and be in rural farmland. Most of the former Western Germany has been densely devloped, with villages every km or so in every direction, for centuries. Former East Germany has some semi-empty areas, but the western states are mostly very dense.